In telecommunications, forward error correction (FEC) is commonly used to detect and correct errors caused by transmission over communication channels. Generally for FEC, a function is used to redundantly encode data to be transmitted over a communication channel. Reed-Solomon (RS) codes are commonly used error correcting codes used in FEC that are generated based on univariate polynomials over finite fields. The redundantly encoded data of the FEC allows the receiver to detect errors in the received data and correct these errors without retransmission.
In optical fiber communications, a transponder is the element that sends and receives the optical signal to/from fiber optic lines. A transponder may be characterized by its data rate and the maximum distance the signal can travel. The transponder may be connected between an electrical telecommunication system and an optical telecommunications system, and includes a host interface connected to the electrical telecommunication system and a line interface connected to the optical telecommunications system.
These type of optoelectrical transponders include multiplexing/de-multiplexing and encoding to convert data received from the lower speed lanes of the host interface to higher speed lanes of the optical interface. An example is a 100 gigabit per second (Gbps) transponder with a host interface including ten electrically encoded 10 Gbps lanes and a line interface including four 25 Gbps optically encoded wavelength lanes.
To use FEC with these transponders, the FEC codes should be applied directly to the line interface so the FEC codes do not get scrambled by the multiplexing and the transmission delay caused by the transponder converting data received from the lower speed lanes of the host interface for transmission on the higher speed lanes of the optical interface. However, the host transmitting data with FEC codes does not have direct access to the line interface of the transponder. Thus, when the host sends data, including FEC codes, to the transponder for transmission over the optical network at high speeds to a destination, the transponder scrambles the FEC codes, which renders them useless for error correction at the receiver. Thus, FEC typically is not used with these type of transponders, and receivers cannot rely on FEC codes for error correction.